The present invention generally relates to a silencer for deadening sound produced from a noise source by the effect of sound wave interference, and more particularly to such a silencer suitable for the case where the noise source is enclosed in a compartment.
Almost every home is generally furnished with a refrigeration system such as a refrigerator, which is in continuous operation throughout the year. Such a household refrigerator can be noisy. In the refrigerator, one critical noise source is a machine compartment enclosing a compressor and piping system connected to the compressor. More specifically, from the machine compartment emanates a relatively loud noise, for example, noise produced from driving a compressor motor, noise produced from the flow of the compressed gas, and mechanical noise produced by movable members of a compression mechanism. Further, the piping system connected to the compressor produces noise due to vibration thereof. The noise emanating from the machine compartment thus accounts for a large part of the noise of the refrigerator. Accordingly, control of noise from the machine compartment contributes to noise reduction in the refrigerator.
Conventionally, compressors of the low noise type such as a rotary compressor have been employed for the purpose of reducing noise emanating from the machine compartment. Further, the construction of vibration-proofing of the compressor has been improved and the configuration of the piping has been improved, thereby providing damping of the vibration in a vibration transmission path. Further, noise absorptive and insulative members have been disposed around the compressor and piping system, thereby improving an amount of noise absorbed in the machine compartment and a noise transmission loss.
However, a plurality of ventilating openings are formed in one or more walls defining the machine compartment for ventilating the machine compartment, and the noise produced in the machine compartment leaks outward through the ventilating openings. As the result of the provision of the ventilating openings, the above-mentioned conventional noise-reduction methods each have a definite limit and provide at most noise reduction of 2 dB.
With the advancement of applied electronic techniques including sound data processing circuitry and acoustic control techniques, application of a system wherein noise is deadened by the effect of sound wave interference has recently been taken into consideration. More specifically, in the above-mentioned noise control, sound generated by a noise source is received by a sound receiver such as a microphone disposed in a specific position and the sound receiver generates an electrical signal in accordance with the received sound. The electrical signal is then converted to a control signal by signal converting means. The control signal is supplied to a speaker so that an artificial sound of opposite phase or 180.degree. out of phase with the noise received by the microphone and having the same frequencies and amplitude as that of the received sound is produced by the speaker, so that the artificial sound interferes with the received sound, thereby deadening the sound. However, when such a noise control system is employed for reducing noise in the machine compartment of the refrigerator, there is a problem in that the noise control manner is complicated since the machine compartment is not completely sealed such that the noise produced in the machine compartment leaks in all directions. Accordingly, in the actual circumstances, the above-described noise control has not been put into practical use in a refrigerator.